Endurance athletes are risking death by drinking excessive amounts of a substance that causes brain cells to swell.
According to the Mayo Clinic, drinking excessive amounts of the substance dilutes the sodium content of the athlete’s blood. This can lead to rapid and dangerous swelling of the brain. A recent article in the New York Times reported that "The engorged brain cells, with no room to expand, press against the skull and can compress the brain stem, which controls vital functions like breathing. The result can be fatal."
The name of the deadly substance?
USA Track and Field is now advising long-distance runners to avoid drinking too much water during events like marathons. Instead of trying to stay ahead of their thirst, runners should use thirst as a signal to drink.
In ordinary circumstances there’s little risk that people will drink so much water that they’ll die. But some are probably drinking more than they need to stay healthy. According to a 2002 media release from the American Physiological Society, most Americans don’t need to the drink 8 glasses of water a day dictated by health folklore (and water company marketers). Dartmouth Medical School’s Heinz Valtin questioned the myth in a review article for the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology (pdf).